A fuel cell is an electrochemical energy conversion device: it transforms chemical power into electrical power. A fuel cell typically converts hydrogen, H2, and oxygen, O2, into water, H2O, producing electricity and heat. A fuel cell provides a direct current (DC) voltage that can be used to power motors, lights or any number of electrical appliances. Some types of fuel cells show promise for use in power generation plants. Others may be useful for small portable applications or for powering cars.
A single fuel cell typically generates a tiny amount of direct current (DC) electricity. In practice, many fuel cells are often assembled into a fuel cell stack. It is estimated that fuel-cell-powered cars will start to replace gas- and diesel-engine cars in about 2005. A fuel-cell car will be very similar to an electric car but with a fuel cell and reformer (for generating hydrogen) instead of batteries. Fuel cells will also be used in portable electronics such as laptop computers, cellular phones and hearing aids. In these applications, the fuel cell will provide longer life than a battery would, and will be “rechargeable” with a liquid or gaseous fuel.
FIG. 1 is an illustration of an exemplary fuel cell. A fuel cell comprises a pair of electrodes (anode 110 and cathode 140) and an electrolyte 120. The electrolyte 120 is typically positioned between the electrodes 110, 140. The electrolyte 120 functions as a conductor for carrying protons 130 between the electrodes 110, 140. In operation, a fuel, such as hydrogen 100, is fed into the anode 110 and oxygen 150 is fed into the cathode 140. The hydrogen 100 atoms, reacting with a catalyst 115 in the anode 110, split into protons 130 (which carry a positive charge) and electrons 160 (which carry a negative charge). The protons 130 are permitted to pass through the electrolyte 120 while the electrons 160 are not. Meanwhile oxygen 150 reacts with a catalyst 135 in the cathode 140 that splits the oxygen molecule 150 into two separate oxygen atoms bearing negative charges. The protons 130 pass through the electrolyte 120 towards the oxygen 150 in the cathode 140. The result is a build up of negative charge in the anode 110 due to the electrons 160 that were left behind. The electric potential generated by this build up of electrons 160 is used to supply electrical power. Often, a fuel cell is equipped with a fuel reformer (not pictured here) that provides hydrogen 100 from a fuel source, such as natural gas, methanol, gasoline, or the like.
There are many different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by the type of electrolyte they use. For example, Phosphoric Acid Fuel Cells (PAFCs) use phosphoric acid as an electrolyte, Molten Carbonate Fuel Cells (MCFCs) use molten carbonate as an electrolyte, Solid Oxide Fuel Cells (SOFCs) typically use a solid zirconium oxide and a small amount of ytrria as an electrolyte, and so on.
The ability of the various electrolytes to conduct protons from one side of the fuel cell to the other is temperature sensitive. Within the proper temperature range, an electrolyte will conduct protons (generally referred to as fuel ions) expeditiously, allowing the electrochemical process to occur rapidly, and thereby producing a greater electrical output. Below the proper temperature ranges, electrolytes will not conduct the fuel ions expeditiously, and the fuel cell will have less than ideal electrical output.
The proper temperature range for the fuel cell depends on the type of electrolyte involved. Some examples of proper operating temperatures for various types of fuel cells (including PAFCs, Proton Exchange Membranes (PEMs), MCFCs, SOFCs, Alkaline Fuel Cells (AFCs), Direct Methanol Fuel Cells (DMFCs) and the Protonic Ceramic Fuel Cells (PCFCs)) are listed below:
PAFC:150°-200°C.PEM:80°C.MCFC:650°C.SOFC:1000°C.AFC:150°-200°C.DMFC:50°-100°C.PCFC:700°C.
Due to the high operating temperatures of fuel cells, it is often necessary to heat them both prior to and during operation. Presently, fuel cells are mostly used in experimental environments, where heat is supplied by holding the cell over a flame or other source of heat. Heat may also be provided by preheating the oxygen that flows into the fuel cell. This requires an air preheat zone where oxygen can be heated prior to entry into the fuel cell. Such an arrangement expedites the heating of the cell or stack, and works well where there is plenty of space, such as in a lab or power plant. The above solutions are insufficient, however, if fuel cells requiring high operating temperatures are to be placed in limited space environments, such as inside electronic devices.
In addition to the standard methods for supplying heat to a fuel cell, the chemical reactions occurring in the fuel cell generate “waste heat” that is useful in keeping the fuel cell or stack of fuel cells hot. Waste heat is simply the heat given off by the chemical reaction of hydrogen and oxygen in the fuel cell. In the case of PEM type fuel cells, which operate at comparatively low temperatures, the fuel cell itself is capable of generating enough waste heat to maintain operating temperature. MEDIS TECHNOLOGIES® has developed an alkaline fuel cell that operates at room temperature. Most other fuel cell types require heat from an outside source, initially to establish the operating temperature of the fuel cell, and in some cases, to subsequently maintain the temperature of the cell. Supplying heat to these fuel cell types in limited space environments presents a challenge to the industry.
SOFCs, in particular, have very high operating temperatures (see above). SOFCs have some promise for use in electronic devices because the electrolyte material they employ is a solid, which lends itself to easier manufacture of small and durable fuel cells. However, the high operating temperature that they require presents a challenge to their eventual implementation in electronic devices. Historically, these fuel cells have been considered best suited for large-scale stationary power generators that could provide electricity for factories or towns. In fact, the high operating temperature of the SOFC has been capitalized on by using the heat to boil water. The steam produced can be channeled into turbines to generate more electricity.
If fuel cells, especially those requiring high operating temperatures, are to be used in limited space environments, there is a need to heat them with a system that requires little space and is controllable. Such a system must be sensitive to the requirements of the environment in which it is placed; if in an electronic device, it must not interfere with the operation of the device. It must also be sensitive to the safety of the device users. A heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies in heating fuel cells for specialized environments, such as may be found within computing devices.